Wide ultra high molecular weight polyethylene sheet and method of manufacture

ABSTRACT

A wide sheet of highly oriented ultra high molecular weight polyethylene comprising a plurality of strips of highly oriented ultra high molecular weight polyethylene partially overlapped or abutted longitudinally to define joints between adjoining strips wherein the thickness of the joint is less than about 80% of the thickness of the sum of the thicknesses of the adjoining strips that make up the joint. A continuous method for the production of such materials comprising subjecting longitudinally overlapping or abutted strips of these materials to temperatures below the melting point of the UHMWPE and pressures over 300 pli is also disclosed.

FIELD OF THE INVENTION

The present invention relates to ultra high molecular weightpolyethylene materials and more particularly to a method for theproduction of wide sheet comprising such materials and the sheetproducts produced by this method.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,951,685 issued Oct. 4, 2005 describes a method for themanufacture of ultra high molecular weight polyethylene (UHMWPE)materials in the form of slit film fibers, tapes and narrow sheets. Suchmaterials are described as having, among other useful propertiesballistic resistance of a very high order.

As is apparent to the skilled artisan, the equipment and processingtechniques described in this patent require significant capitalinvestment and the application of relatively stringent processingconditions. Both of these requirements increase virtually exponentiallyas the UHMWPE product width is increased from a fiber to a tape andupwards to a sheet. Thus, in order to contain the additional cost ofequipment required to make such wider materials, i.e. sheet as definedherein, it would be desirable to have a method for their manufacturethat minimizes such costs and process control requirements.

U.S. Pat. No. 4,931,126 describes an apparatus for increasing the widthof a fiber reinforced thermoplastic sheet or tape product, whichapparatus increases such width by longitudinally joining parallel sheetsor tapes of the fiber reinforced thermoplastic material in an overlap orbutt configuration and melting the overlapping or abutting areas of theparallel tapes.

European Patent Publication No. EP 1 627 719 A1 describes a multilayeredUHMWPE material comprising a plurality of “monolayers” of UHMWPE in theabsence of any adhesive wherein the each monolayer is laid at an angleto any adjacent monolayer. The term “monolayer” as used in thispublication is defined as comprising “a plurality of high-strengthunidirectional polyethylene strips, oriented in parallel in one plane,next to one another”. According to one embodiment the strips partiallyoverlap. The “monolayers” are formed by subjecting the overlying stripsto conditions of temperature and pressure in the ranges of 110-150° C.and 10-100 N/cm² These conditions produce a “sheet” having joint areasthat are inadequate to maintain even a modicum of integrity and theirproperties are grossly inferior to those of the sheets of the presentinvention, as will be demonstrated in the discussion and examples thatfollow.

There thus remains a need for a method of producing wide strips orsheets of substantially pure and highly oriented UHMWPE from narrowertapes or strips of these materials, and for the products produced bysuch a method.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor the production of wide sheets of any custom width from narrow stripsor tapes of substantially pure and highly oriented UHMWPE, which widesheets exhibit properties equal or superior to those of the parent stripmaterials from which the wide sheets were fabricated.

It is another object of the present invention to provide a wide sheet ofsubstantially pure and highly oriented UHMWPE, which wide sheets exhibitstrength and modulus properties equal or superior to those of the parentstrip materials from which the wide sheets were fabricated.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of the apparatus useful in thefabrication of the wide sheet ballistic materials described herein.

FIG. 2 is a schematic top view of the apparatus useful in thefabrication of the wide sheet ballistic materials described herein.

FIG. 3 is a schematic side view of the calendar roll stand portion ofthe apparatus useful in the fabrication of the wide sheet ballisticmaterials described herein.

FIG. 4 is a schematic partial top view of a portion of the initial andfinal alignment guide zones of the apparatus useful in the fabricationof the wide sheet ballistic materials described herein.

FIG. 5 is a cross-sectional view of the overlap area between twoadjoining narrow strips of highly oriented UHMWPE prior to bonding inaccordance with the practice of the present invention.

FIG. 6 is a cross-sectional representation of the joint volume betweentwo highly oriented UHMWPE tapes after treatment in accordance with themethod of the present invention.

FIG. 7 is a partially phantom top plan view of the UHMWPE wide sheet ofthe present invention prior to processing in accordance with the methodof the present invention.

FIGS. 8 and 9 are cross-sectional views depicting an alternativepreferred embodiment of the present invention.

SUMMARY OF THE INVENTION

According to the present invention there is provided a wide sheet ofhighly oriented ultra high molecular weight polyethylene comprising aplurality of strips of highly oriented ultra high molecular weightpolyethylene of indeterminate length abutting or partially overlappedlongitudinally to define joints between adjoining strips and wherein thethickness of the joint is less than about 80% of the thickness of thesum of the thicknesses of the adjoining strips that make up the joint. Acontinuous method for the production of such materials comprisingsubjecting longitudinally overlapping strips of these materials totemperatures below the melting point of the UHMWPE and pressures over300 pli as well as the apparatus useful in the process of the presentinvention are also disclosed.

Detailed Description

As used herein, the terms “substantially flat”, “essentially flat” and“substantially pure” are meant and intended to have the followingmeanings: “substantially flat” refers to sheet material in accordancewith the present invention wherein a joint between two adjoining stripsof material has a thickness that is not greater than 80% of the combinedthicknesses of the adjoining/overlapping/abutting strips; “essentiallyflat” refers to sheet material in accordance with the present inventionwherein a joint between two adjoining strips is essentially the samethickness as that of the strips being joined with little if anythickness difference therebetween; and “substantially pure” refers toUHMWPE that contains no foreign materials or substances that negativelyaffect the properties of the UHMWPE except as artifacts of the UHMWPEproduction process such as catalysts, etc.

The starting material UHMWPE strips of the present invention are thoseprepared in accordance with the methods described in the following U.S.Pat. Nos. 6,951,685; 4,879076; 5,091,133; 5,106,555; 5,106,558; and5,578,373 the teachings of which are all incorporated herein byreference in their entireties. Particularly preferred as the startingmaterials in the process described herein are the UHMWPE materialsprepared as described in the aforementioned U.S. Pat. No. 6,951,685.Such material comprise highly oriented UHMWPE of high purity.

According to the process of the present invention, wide UHMWPE sheet isproduced by a process that comprises calendering an array of overlappingor abutting strips of indeterminate length prepared as just described ata temperature below the melting point of the UHMWPE, generally in arange of between about 120° C. and about 155° C. (depending upon thetension applied to the strips during bonding as described below) at apressure above about 300 pounds per lineal inch (pli) and under atension of between about 0.3 grams/denier and about 5 grams/denier. Thearrangement of the array and the resulting final product sheet isdepicted in FIG. 7. The calendering apparatus used to accomplish theprocess is depicted in FIGS. 1-3.

Referring now to accompanying FIGS. 5-7, a first embodiment of the wideUHMWPE sheet of the present invention 300 comprises a series of paralleland overlapped tapes or strips 302 of indeterminate length. As usedherein, in relation to this first preferred embodiment, the term “joint”is meant to define and refer to the overlapped areas/volumes 304depicted n FIG. 6. As depicted in FIGS. 5, 6, 10 and 11, the moleculesin two abutting or overlapping strips or tapes 302A and 302B areschematically depicted as triangles and circles to permitdifferentiation in the discussion that follows.

As depicted in FIG. 5, 6 and 7, a first preferred embodiment of the widesheet 300 of the present invention is produced by overlaying an array oftapes or strips 302A, 302B etc. of whatever width in parallellongitudinal relationship and then subjecting them to the processingconditions in the apparatus described herein. As shown in FIG. 5, in oneembodiment of the present invention, each of overlaying strips or tapes302A and 302B is 0.0025 inches in thickness and the molecules(schematically represented as triangles and circles) are in each ofseparate strips or tapes 302A and 302B. As shown in FIG. 6, once theoverlapping structure has been subjected to the process conditionsdescribed herein, the total thickness of the joint 304 is about 0.0032inches, a total reduction of more than about 35% and the molecules havebeen intermingled, in this case most probably entangled to provide ajoint 304 that exhibits a higher strength than the parent material aswell as a higher modulus. The thicknesses of strips or tapes 302A and302B just mentioned are used for demonstration purposes only, it beingclearly contemplated that thicker or thinner strips 302A and 320B couldbe equally well used to for the UHMWPE wide sheet described herein. Moreparticularly, strips having thicknesses between about 0.0010 inches and0.01 inches, for example, could be equally well used to form the widesheet of the present invention assuming the availability of suitablecalendaring equipment. Strips in the range of between about 0.0015 andabout 0.007 inches in thickness are specifically preferred for use inaccordance with the present invention. It should be noted that suchthickness reduction in joint area 304 and the intermingling of themolecules of each of the parent strips or tapes 302A and 302B can onlybe accomplished with the application of the pressures described herein.Subjection of the overlapping structure to lower pressures, as describedin the prior art, does not achieve the thickness reduction and molecularcommingling of the present invention or the strength and modulusincreases resulting therefrom. The attainment of these enhancements andtheir presence clearly and unequivocally distinguish the process andproducts of the present invention from those of the prior art. Theseenhancements are demonstrated in the discussion that follows inconnection with FIGS. 8 and 9.

FIGS. 10 and 11 depict cross-sectional views representing an alternativepreferred embodiment of the UHMWPE wide sheet of the present invention.As shown in FIG. 10, according to this embodiment, two strips 302A and302B of UHMWPE are butted together. The processing of this buttedconfiguration under the processing conditions described herein and inthe apparatus described herein results in the structure shown in FIG. 11wherein each of strips 302A and 302B has undergone a degree of “sideextrusion”, i.e. the longitudinal edges of each of the strips has beenblended with the longitudinal edge of the abutting strip to form a jointarea/volume 304 defined by the merger of the molecules of each of themember strips depicted as circles and triangles for differentiationpurposes in these two Figures. This product wide sheet is fabricated bylaying up an array of longitudinally abutting strips of UHMWPE andsubjecting the array thus formed to the processing conditions describedherein in an apparatus similar to that described above with theexception that instead of overlaying neighboring strips of UHMWPE thestrips are butted against each other prior to processing. Under theseconditions, the abutting strips undergo side extrusion forcing theneighboring edges into each other to provide the structure depicted inFIG. 11. As can be envisioned and as depicted in FIG. 11, this widesheet comprises an essentially flat sheet with little or no thicknessdifference in joint area/volume 304.

Referring now to FIGS. 1-3, the apparatus utilized in accordance withthe present invention to obtain the superior wide ballistic sheet of thepresent invention comprises seven discrete zones 10-70 as depicted inFIG. 1. Zone 10 is the feedstock payoff zone, zone 20 comprises atension control zone that helps develop tension (other means are ofcourse possible such as the inclusion of additional rollers), zone 30 isthe initial and final alignment guide zone, zone 40 is a motor drivenroll stand that imparts pulling energy to draw material throughapparatus 1, zone 50 comprises the calender rolls that apply heat andpressure to bond the strips 01 of overlapped material, zone 60 comprisesa motor driven roll stand that pulls the overlapped material from thecalender and feeds it to the take up stand or zone 70.

Individual rolls of material 01 and 01′ (shown as element 302 in FIGS.5, 6 and 7) are mounted on shafts 12 and 12′ to support them forunrolling and to place them in staggered relationship. The material oneach of individual rolls 1 has an edge 3 and the edges 03 on staggeredrolls 01 and 01′ are oriented so as to overlap slightly as shown in theaccompanying Figures. A resistance mechanism 14 is applied to rolls 1 tocontrol their rate of unwinding.

As material 302 exits feedstock payoff zone 10 it is passed through aseries of bars 20 (best seen in FIG. 1) that serve to control tension asmaterial 302 is pulled through the line by subsequent operations. Aswill be explained more fully below, tension control is very important tothe successful practice of the present invention.

Upon exiting zone 20 material 302 enters zone 30 which comprises twosets of offset rolls 31 and 31′ that include flanges 32 and 32′ mountedupon adjustable shafts 33 and 33′ that serve to direct the flow ofmaterial 302 into subsequent zone 40 and control the amount of overlapof material 302 as it enters this subsequent zone.

Zone 40 comprises a series of vertically offset rolls 40 and 40′ thatpull material 302 from feedstock rolls 01 and through zones 20 and 30. Amotor 42 is provided to drive rolls 41 and 41′.

Zone 50 comprises a final set of guide rolls 31 including flanges 32mounted on a shaft 33 which serve to provide final guidance ofoverlapped material 302 into calender zone 50. The overlapped materialsat this point in the process and in accordance with this embodiment areshown generally in FIG. 4. As shown in this Figure three input strips 1of widths W1, W2 and W3 are overlapped a distance WT. WT may vary widelyfrom a small fraction of an inch upwards to an inch or two. The amountof overlap is not particularly significant and does not materiallyaffect the process or the product produced thereby. Within calender zone50 are located calender rolls 51 and 51′ that supply the requisitepressure to overlapped material 302 as specified elsewhere herein andexiting zone 50 is wide ballistic sheet 300 comprising overlapped andintimately bound sections of material 302 as shown in FIG. 5. Asdepicted in FIG. 3, a lift bar 55 driven by cylinder 54 is provided tolift top roll 51 to permit threading of overlapped material 302 betweencalender rolls 51 and 51′.

After exiting zone 50 wide ballistic sheet 300 enters zone 60 whichcomprises an offset set of pull rolls 61 which serve to draw materialthrough apparatus 100 under tension as described elsewhere hererin. Amotor 62 is provided to drive rolls 60.

In zone 70 wide ballistic sheet 300 is taken up and rerolled onto ashaft 71 driven by motor 72.

Referring now to FIG. 4, it can be seen that as material 01 enters thevarious guide rolls described hereinabove and more specifically guiderolls 31 proximate calender rolls 51 and 51′, each has a specific widthW1, W2 or W3 which are preferably all the same but could be different,and overlap as shown in FIG. 4 and also shown in greater detail in FIG.7.

The processing conditions described herein, temperatures below themelting point of the UHMWPE strips, tensions in the range of from about0.3 and about 5 grams/denier and pressures above about 300 pli, definean operating window whose parameters of temperature and tension areintimately interrelated. As is well known in the art of producingUHMWPE, as tension on a fiber or strip of UHMWPE the “melting point”i.e. the temperature at which the onset of melt can be detected,increases as tension increases on a fiber or strip. Thus while at atension of 0.3 grams/denier a temperature of about 120° C. may be belowthe melt point of the UHMWPE strips, at a tension of 5 grams/denier atemperature of 154° C. may still be just below the melt point of theUHMWPE strips. Thus, this interrelationship of tension and temperaturemust be carefully considered and maintained in order to obtain theenhanced products of the present invention. The pressure element of theprocessing conditions, is largely independent of the tension andtemperature relationship just described. According to various preferredembodiments of the processing conditions of the present invention,temperatures in the range of from about 125° C. and 150° C. and tensionsin the range of from about 0.4 and about 4.5 grams/denier arespecifically preferred. The speed at which the process can be operatedsuccessfully is dependent solely upon the rate at which heat can beimparted to the UHMWPE strips. As long as the strips can be brought tothe proper temperature prior to introduction into the calender rolls,the process will be effective. Such more rapid heating could be throughthe use of a preheating oven, the use of larger calender rolls, multiplesets of calender rolls, the use of multiple calenders, etc.

UHMWPE wide sheet produced in accordance with the process describedherein exhibit a remarkable degree of transparency, in excess of 30%,while those of the prior art prepared as described below exhibited theopacity of the parent strip materials. This is undoubtedly due to eitherthe fact that at low temperatures the process of the prior art does notproduce well consolidated or intimately commingled structures, thus,exhibiting the transparency of the parent material, while at highertemperatures melting occurs, as discussed in greater detail below,leading to the presence of voids in the melted areas that serve todiffuse light and result in increased opacity.

In order to demonstrate clearly the distinctions between the products ofthe present invention and the far inferior products of the prior art,samples of wide UHMWPE fabricated in accordance with the presentinvention and in accordance with the process described in EuropeanPatent Publication No. EP 1 627 719 A1 were produced and subjected toSEM study to clearly observe the structural differences between thejoint areas/volumes in each of the products. The results of thesestudies are discussed below.

SEM (scanning electron microscope) images made across a joint in each ofthe products in the direction shown by arrow A-A in FIG. 7, i.e.transverse to the length of joint area/volume 304 of a joint made inaccordance with the processing parameters described in theaforementioned European Patent Publication No. EP 1 627 719 A1(processing conditions used to fabricate this sample were specifically atemperature of 110° C. and a laminating pressure of 145 psi) clearlyshows a distinct “joint line”, i.e. a point in the joint area/volumewhere the materials have not been intimately blended. This joint lineserves as an indication that intimate blending of the material from thetwo strips that form the joint was not obtained. Testing of this jointshowed that it peeled apart easily and retained virtually no structuralintegrity when subjected to separating forces.

An SEM photograph of a sample of wide UHMWPE sheet fabricated inaccordance with the present invention shows that there is no “jointline” and the point at which the materials from the overlapping sheetsmeets is indistinguishable from the parent materials. This joint wasvirtually impossible to separate and at this time appears to exhibitstrength and modulus properties superior to those of the parent stripmaterial.

In further evaluation of the teachings of the prior art, samples wereprepared according to the teachings of the reference at temperatures of140° C. and 150° C. and pressures of 145 psi and 14.5 psi respectively.A study of photomicrographs of these joints shows that there is no jointline in these samples, however, in these instances, the absence of adistinct joint line is due to melting of the UHMWPE strips in the jointarea as shown by the residual striations or voids apparent in thephotomicrographs. It is well known that the UHMWPE materials utilized inthe prior art exhibit what is characterized as the “onset of melt”(these materials do not exhibit a clear and distinct melting point) inthe range of about 140° C. Thus, in spite of the continued teachings ofthe prior art that lamination should occur below the melting point ofthe UHMWPE material, the process only produces an integrated structurewhen practiced above the melting point of the parent material. Meltingof the UHMWPE in a sense “anneals” the material thereby significantlyreducing its modulus and strength as compared to an “unannealed” bondedmaterial.

In order to make the comparison of the processing conditions of thepresent invention and those of the prior art more direct, it has beencalculated that the pressures utilized in the present invention areabove about 17,000 psi and about 85,000 psi at the upper end of thedescribed useful pressures. These pressures are considerably higher thanthe 14.5-145 psi pressures indicated as useful in the prior art EuropeanPatent Publication. Stated more comparatively the pressures used in thepresent process are generally in excess of about 2000 N/cm as opposed tothe 10-100 N/cm² taught by the prior art. Thus, the process of thepresent invention produces an UHMWPE wide sheet that is considerablydifferent than that produced by the prior art process.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

1) A wide sheet of ultra high molecular weight polyethylene comprising aplurality of strips of highly oriented ultra high molecular weightpolyethylene partially overlapped or abutted to define joints betweenadjoining strips wherein the thickness of the joint is less than about80% of the thickness of the sum of the thicknesses of the adjoiningstrips that make up the joint. 2) The wide sheet of ultra high molecularweight polyethylene of claim 1 wherein the thickness of the joint isless than about 60% of the thickness of the sum of the thicknesses ofthe adjoining strips that make up the joint. 3) The wide sheet of ultrahigh molecular weight polyethylene of claim 1 wherein the thickness ofthe joint is equal to the thickness of the thicknesses of the adjoiningstrips that make up the joint. 4) The wide sheet of ultra high molecularweight polyethylene of claim 2 wherein said joint exhibits atransparency above about 30%. 5) The wide sheet of ultra high molecularweight polyethylene of claim 2 wherein each of the adjoining strips hasa thickness of between about 0.0010 inches and about 0.007 inches. 6)The wide sheet of ultra high molecular weight polyethylene of claim 5wherein each of the adjoining strips has a thickness of about 0.0025inches and the thickness of the joint is below about 0.004 inches. 7) Asubstantially flat high strength, high modulus wide sheet having asubstantially uniform thickness, an indeterminate length and a widewidth made from a plurality of narrow strips of an indeterminate lengthand a narrow width of substantially pure highly oriented ultra highmolecular weight polyethylene partially overlapped or abutted withadjacent parallel narrow strips in the oriented direction such that theoverlapped or abutted portions of narrow strips are forced into orsideways into one another to form an intermingled joint having athickness substantially the same as the thickness of the non-overlappingor abutted portions of the narrow strips. 8) A method for the productionof a wide polymeric sheet comprising: A) placing a series of elongatedstrips of a polymer in overlapping or abutting longitudinal relationshipto form an array of polymeric strips; B) applying tension to the arrayof polymeric strips; and C) calendering the array of polymeric strips ata temperature below the melting point of the polymer. 9) The method ofclaim 8 wherein the polymer is ultra high molecular weight polyethylene,tension of between about 0.3 grams/denier and about 5 grams/denier isapplied to the array of strips and calendering is performed at atemperature of between about 120° C. and about 154° C. and a pressureabove about 300 pli. 10) The method of claim 9 wherein the tension isbetween about 0.4 grams/denier and about 4.5 grams/denier, thetemperature is between about 123° C. and about 153° C. and the pressureis above about 340 pli. 11) A method for the production of wide sheetsof ultra high molecular weight polyethylene comprising calendering anarray of longitudinally overlapping or abutted strips or tapes of ultrahigh molecular weight polyethylene at a pressure of at least about 300pli, a temperature of between about 120° C. and 155° C. and under atension of from about 0.3 to about 5 grams/denier.